Cam drives are usually used to generate the reciprocating movement of the auxiliary grippers of sheet-fed printing machines. In order that the cam follower secured to the control lever may be kept in continual contact with the control cam, the control lever must be subjected to a force which presses the cam follower against the control cam even when it tends to lift away from the cam due to the mass inertia of the drive and of the auxiliary gripper. This is necessary in order to avoid damage to the transmission, reduce vibration in the machine, and ensure accurate-register sheet transport to the printing machine.
German Pat. No. 677,130 describes an auxiliary gripper control system for sheet-fed printing machines. As shown in this disclosure, two disc cams are secured on the printing cylinder shaft and rotate with the printing cylinder in fixed relationship to one another. A guide roller runs on the first disc cam and is mounted on the control lever, which drives the auxiliary gripper via a pull-rod. The control lever is pivotable about a spindle secured to the frame. A second lever is pivotally mounted on the same spindle and a second guide roller is also secured thereon to run on the second cam. The end of the second lever extends beyond the frame mounting and has an extension in the form of a horn with a compression spring disposed between this horn-shaped extension and the first control lever.
In this arrangement, the lobes of the two cams are so adapted to one another that during the auxiliary gripper movement the spring between the two levers is always at the same tension and performs no working movement. This ensures that the guide rollers bear continually against the cam; but, at the same time there are no unnecessary movements in the drive. A disadvantage of this arrangement, however, is that the two cam discs are situated in two parallel planes transverse to the printing cylinder axis. The control lever and the second lever are thus also in two different planes. Consequently, although no relative movements are provided between the two levers, the prestressing forces applied by the compression spring and the acceleration forces resulting from these movements have to be transmitted via the joint spindle of the two levers. Also, valuable space is taken up by the arrangement of the parallel cams disposed axially one behind the other on the printing cylinder shafts. This is a disadvantage because the printing unit drive gear trains also have to be accommodated in this area.